The invention concerns a power supply which converts DC power to AC output power, in which the shape of the waveform of the output, as well as its frequency, can be controlled.
FIG. 1 shows a simplified xe2x80x9cbuckxe2x80x9d type voltage regulator. Such regulators provide very high performance in terms of constant output voltage, low output ripple voltage, and controlled transient behavior. These performance advantages are somewhat offset by the facts that they can be heavy and bulky. These regulators are used to convert a high DC voltage into a lower DC voltage.
The regulator alternates between the two conditions shown in FIG. 1A and FIG. 1B. In FIG. 1A, the switch S is closed, and a charging current ICHARGE flows through the inductor L. In FIG. 1B, the switch S is open, and a discharging current IDISCH flows through diode D. If it is assumed that capacitor C is large, so that VC remains constant, and that the switch S and diode D are ideal elements, then FIG. 2 illustrates the approximate behavior of the apparatus of FIGS. 1A and 1B.
FIG. 2A illustrates the time-behavior of switch S. A duty cycle D is defined as the ratio of t1/T, as indicated. FIG. 2B illustrates the current IL through the inductor L. The current remains positive at all times, and swings from IL1 to IL2, and back. FIG. 2C illustrates the voltage VL across the inductor. It swings from a positive value of (V1xe2x88x92VC) to a negative value of (xe2x88x92VC).
FIG. 2D illustrates the voltage across diode D. When the switch S is closed, V1 appears across the diode D. When the switch S is open, a zero voltage appears across the diode, because the diode is assumed to be ideal.
FIG. 2E illustrates the voltage across the switch S. In a sense, this voltage is opposite to that across the diode D. When the switch S is open, V1 appears across the switch S. When the switch S is closed, a zero voltage appears across the switch S.
FIG. 2F illustrates the current Ic flowing into the capacitor. When the switch S is closed, the capacitor C charges. When the switch S opens, inductor L is placed in parallel with the capacitor C, but with a negative voltage, thereby causing the current charging the capacitor C to diminish, in region R1 in FIG. 2F, and then to become negative, in region R2, which represents a discharge of capacitor C. Then, when the switch S closes again, in region R3, the capacitor C charges again. The hatched regions represent equal charges, because current multiplied by time equals charge.
The plots of FIG. 2 are justified by the following equations. The variables are defined in
FIGS. 1A, 1B and 2.
V1=constant
VC=average xe2x80x9conxe2x80x9d time of switch=D*V1
VL=L dIL/dt
Switch off:
VC=L(DEL IL)/(DEL T)=L(IL2xe2x88x92IL1)/t1xe2x86x92IL2xe2x88x92IL1=VCt1/L
Switch on:
V1xe2x88x92VC=L(DEL IL)/(DEL T)=L(IL2xe2x88x92IL1)/t2xe2x86x92IL2xe2x88x92IL1=t2(V1xe2x88x92VC)/L
The preceding four equations justify FIG. 2B. FIG. 2C is based on an application of Kirchoff""s Voltage Law (KVL) to FIGS. 1A and 1B, and the stipulation that VC is constant. FIGS. 2D and 2E are also based on KVL.
FIG. 2F is based on the fact that the steady-state average of IC must be zero.
IR=V2/R
IL(avg)=(IL1+IL2)/2=V2/Rxe2x86x92IL2+IL1=2V2/R
Combining the last with the xe2x80x9cswitch onxe2x80x9d equations gives
IL2=V2(IL(avg)/R+t1/2L)
IL1=V2(IL(avg)/Rxe2x88x92t1/2L)
The capacitor ripple current is computed by
IC=ILxe2x88x92VC/R
This equation justifies the shape of the plot in FIG. 2F: it is the plot of FIG. 2B, minus a constant, namely VC/R. The following two equations determine the vertical position of the plot of FIG. 2F.
IC2=IL2xe2x88x92(IL1+IL2)/2=(IL2xe2x88x92IL1)/2
IC1=IL1xe2x88x92(IL1+IL2)/2=xe2x88x92(IL2xe2x88x92IL1)/2
It can be shown that, if the inductance L has an internal resistance r, then VC equals V1 multiplied by the factor [R/(R+r)]xc3x97D. V1 has been reduced. The buck regulator acts as a voltage reducer.
The regulator just described provides a DC output, with a small ripple. In some situations, an AC output is desired.
An object of the invention is to provide an improved power converter.
In one aspect, this invention comprises a switching power supply, which contains a switch which pulses current through an inductor, the improvement comprising means for measuring a current controlled by the switch and means for comparing that current with a time-varying reference and, at the time of comparison, if the current exceeds the reference, opening the switch.
In another aspect, this invention comprises method of operating a switching power supply, comprising the steps of applying a voltage to an inductor-resistor combination, to generate an increasing current in the combination, comparing a signal indicating the current with a time-varying reference and if the current attains a predetermined relation to the reference, removing said voltage from the combination.
In still another aspect, this invention comprises a switching power supply improvement comprising means for generating a reference AC waveform and means for generating an output waveform which is a magnified replica of the reference waveform.
In yet another aspect, this invention comprises a power supply which receives an input waveform and supplies current to a load (R) comprising a capacitor in parallel with the load (R), an inductor in series with the capacitor, a first switch (IGBT1), which sources current to the inductor, when closed, a second switch (IGBT2), which sinks current from the inductor, when closed, a first flip-flop (FF1), whose output controls closure of the first switch (IGBT1), a second flip-flop (FF2), whose output controls closure of the second switch (IGBT2), a clock connected to the set (S) inputs of both flip-flops, first and second current measurement means (HALL1, HALL2) for measuring current through the respective first and second switches (IGBT1, IGBT2), a first comparator (COMP1) for comparing measured current through the first switch (IGBT1) with the input waveform and, when the former exceeds the latter, applying a reset signal to the first flip-flop (FF1), to thereby open the first switch (IGBT1), and a second comparator (COMP2) for comparing measured current through the second switch (IGBT2) with the waveform and, when the former exceeds the latter, applying a reset signal to the second flip-flop (FF2), to thereby open the second switch (IGBT1) wherein a signal is generated in the load (R) which approximately replicates the input waveform.
A further object of the invention is to provide a voltage regulator which produces a controllable waveform.
In one form of the invention, a buck regulator is used to provide a high-current, high-voltage replica of a reference waveform.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims.